1. Field of the Invention
This invention relates to the field of communication paths, such as a bus, in a computer system, and the transmission of clock and/or data signals on a bus.
2. Background Art
A computer system consists of a number of modules or components. Computer systems typically include a central processing unit (CPU) such as a microprocessor. The microprocessor is a program controlled device that obtains, decodes and executes instructions. A computer system also includes program storage components for storing program instructions, and data storage components for storing data. These storage components may be read only memory (ROM), random access memory (RAM), disk or tape storage, or any other suitable storage means.
A computer system also includes input/output (I/O) components for connecting external devices to the microprocessor. Special purpose components, such as memory management units or co-processors, may also be part of the computer system.
The various components of a computer system are linked by a communication path referred to as a "bus". The bus is a communication path (such as a set of conductors) used to transfer information such as instructions, data, addresses or control signals between the components of a computer system. The components coupled to the bus can both place a signal onto the bus (signal source) or receive a signal from the bus (signal receiver).
In operation, a signal source, also known as a "driver", asserts a signal on the bus by varying the potential of the bus. The signal source drives the potential on the bus to some predetermined level (high state) and/or pulls the bus to a ground level (low state). Either state of the bus can represent digital information such as logical "1's" or "0's". The signal generated by the driver travels on the bus until it reaches an intended receiver. The receiver, by detecting the state of the bus, receives the information transmitted on the bus.
The performance of the bus, and therefore the performance of the computer system associated with the bus, depends on the "delay time" of signals generated on the bus. The delay time of a signal is a combination of the "propagation delay" (the time it takes a signal to go from the active driver to the furthest end of the bus) and "reflection delay" (the time it takes for any signal reflections to fall below a threshold level, permitting the bus to be driven with a new signal).
For an ideal bus, propagation delay depends only on bus length and the characteristic impedance associated with the bus. In reality, however, propagation delay depends on the number of devices coupled to the bus. Each device coupled to the bus has an associated parasitic capacitance. The effect of this parasitic capacitance is to increase the propagation delay time, lowering bus performance. The more devices coupled to the bus, the greater the effects of the parasitic capacitance and the lower the bus performance.
Reflection delay is also a limiting factor in bus performance. When a signal is driven onto a bus, it travels the length of the bus and is reflected back along the bus toward the source. This reflected signal may be reflected again at the source, resulting in a plurality of reflected signals on the bus. The amplitudes of these reflected signals decay with time until they are no longer detectable. The time it takes a reflected signal to decrease below a predetermined level is known as the settling time or reflection delay.